The production of an effective mathematics curriculum begins with a scientific development, evaluation, and revision framework. The purpose of this study was to conduct an initial investigation of a recently developed Tier 2 mathematics curriculum designed to improve the outcomes of 1st grade students at-risk for mathematics difficulties. The curriculum, which is anchored to a scientific design framework and a well-specified theory of change, centers on the careful integration of foundational concepts of whole number and validated-design principles of explicit and systematic instruction. Four instructional groups, with each consisting of five 1st grade students with mathematics difficulties and one interventionist, participated in the study. Data related to the feasibility, usability, and promise of the curriculum to improve student mathematics achievement were collected using multiple methods. Results provide preliminary evidence in terms of these curriculum development and evaluation areas. Implications for instruction and design research are discussed. [This paper was published in "Learning Disability Quarterly" v38 n2 p97-111 May 2015.]